Base Saturation Calculator
Enter exchangeable Ca, Mg, K, Na and cation exchange capacity CEC to compute base saturation BS% = (sum of bases ÷ CEC) × 100, assessing soil fertility and acidification.
Input Data
Results
At a glance:Enter exchangeable Ca, Mg, K, Na and cation exchange capacity CEC to compute base saturation BS% = (sum of bases ÷ CEC) × 100, assessing soil fertility and acidification.
Formula
BS% = (Ca + Mg + K + Na) ÷ CEC × 100.
$$BS\% = \dfrac{Ca^{2+} + Mg^{2+} + K^+ + Na^+}{CEC} \times 100$$How to Use
- Enter the exchangeable Ca, Mg, K, Na (cmol(+)/kg) from the soil report.
- Enter the soil CEC (cmol(+)/kg).
- The calculator returns the sum of bases and BS%.
FAQ
What is the relationship between base saturation and soil pH?
They are closely related but not identical. The more acidifying H⁺ and Al³⁺ occupy the exchange sites, the lower the BS% and the more acidic the soil; liming adds Ca²⁺ and Mg²⁺ to the sites, displacing H⁺ and Al³⁺, raising both BS% and pH. Generally BS% >80% corresponds to neutral-to-alkaline, <50% to acidic. But the real relationship also depends on buffering capacity, clay minerals, and organic matter, so pH should still be measured directly.
What BS% is good?
There is no absolute standard; it varies with crop and soil. Empirically BS% >80% is mostly fertile neutral soil, 60–80% good, 50–60% moderately acidic, <50% base-deficient and acidic, often needing amendment. Acid-loving crops (e.g. blueberry, tea) actually prefer lower BS% and pH. The key is not just total BS% but whether the Ca, Mg, K balance is appropriate.
Why use cmol(+)/kg instead of mg/kg for the ions?
Because base saturation compares the charge share of exchange sites, and CEC itself is expressed in charge (cmol(+)/kg — how many centimoles of positive charge sites per kg of soil). Different ions have different valences (Ca²⁺, Mg²⁺ divalent; K⁺, Na⁺ monovalent), so they must be converted to equivalent charge to be summed and compared with CEC. If the report gives mg/kg, first divide by each ion's equivalent weight to convert to cmol(+)/kg before entering.
Can the sum of bases exceed CEC?
Theoretically no — bases cannot occupy more sites than exist, so BS% maxes at 100%. If your computed sum of bases exceeds CEC, it is usually analytical error, a different CEC method (e.g. a smaller effective CEC from unbuffered extraction), or interference from soluble salts. Then BS% should be read as near or at 100% (this tool clamps the result within 100%), and you should check data and method consistency.
Is BS% alone enough? What else to look at?
No. BS% reflects the total share of exchange sites occupied by base nutrients, but crop uptake also depends on each nutrient's absolute amount and their ratios. For example, a decent BS% with excessive Mg antagonizes K uptake and causes K deficiency. A full fertility diagnosis also needs soil pH, the Ca:Mg:K ratio, absolute nutrient contents, organic matter, texture, and CEC magnitude (high-CEC soils hold more nutrients and buffer more). BS% is a key piece, not the whole picture.
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References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.